1. Field of the Invention
The present invention relates to a process for the elimination of pyrite in coal, shale and other carbonaceous material. The elimination of pyritic sulfur by the present method avoids the undesirable by products when pyrite containing solids are burned.
2. Discussion of the Prior Art
Coal is an important fuel, and large amounts are burned in thermal generating plants primarily for the production of electrical energy. One of the principal drawbacks in the use of coal as a fuel is that many coals contain amounts of sulfur which generate unacceptable amounts of sulfur oxides upon burning. For example, coal combustion is by far the largest single source of sulfur dioxide pollution in the United States at present and currently accounts for 60 to 65% of total sulfur oxide emissions.
The sulfur content of coal, nearly all of which is emitted as sulfur oxides during combustion, is present in essentially two forms: inorganic, primarily metal pyrites, and organic sulfur. The inorganic sulfur compounds are mainly iron pyrites. The organic sulfur is chemically associated with the coal structure itself. Depending on the particular coal, the sulfur content can be primarily in the form of either inorganic sulfur or organic sulfur. Distribution between the two forms varies widely among various coals. For example, both Appalachian and Eastern interior coal as well as Western and Midwestern coals are known to be rich in pyritic and organic sulfur.
Oil-shale deposits in the United States occur over a wide area with the most extensive deposits in the Devonian-Mississippian shales of the Eastern United States. Additional deposits are present in the Green River formation of Colorado, Utah and Wyoming. These vast deposits offer an important potential reserve of hydrocarbons in the face of the eventual depletion of the conventional sources of oil. Various methods, such as for example, retorting and pyrolsis are used to win oil from shale. Various techniques are used commercially as illustrated in Kirk-Othmer, Vol. 16, pp 333-352 (John Wiley and Sons, New York). However, air pollution in the form of gases containing oxides of sulfur, nitrogen oxides, carbon monoxides and trace hydrocarbons remains an important problem. Additionally, retorting, refining and electric power generation contribute to the pollution problem when coal, shale or other carbonaceous material that contains substantial amounts of pyrite are used as sources of fuel in these processes. Thus, a mechanism to remove pyritic sulfur prior to combustion or retorting of shale is important before effective utilization of these fuels can be exploited.
Heretofore, it was recognized that it would be highly desirable to eliminate (or substantially eliminate) the sulfur content of coal or other carbonaceous materials prior to combustion. In this regard, a number of processes have been suggested in reducing inorganic (pyritic) portion of the sulfur in coal, shale and other carbonaceous materials.
For example, it is known that at least some pyritic sulfur can be physically removed from coal by grinding the coal, and subjecting the ground coal to froth floatation or washing processes. While such processes can desirably remove pyritic sulfur and ash from the coal, these processes are not fully satisfactory because a significant portion of the pyritic sulfur is not removed. Attempts to increase the portion of pyritic sulfur removed have not been successful because the processes are not sufficiently selective and often can result in a large portion of coal being discarded along with ash and pyrite.
There have also been suggestions heretofore to chemically remove pyritic sulfur from coal. For example, U.S. Pat. No. 3,768,988 to Meyers, issued Oct. 30, 1973, discloses a process for reducing the pyritic sulfur content of coal which involves exposing coal particles to a solution of ferric chloride. The patent suggests that in this process ferric chloride reacts with pyritic sulfur to provide free sulfur according to the following reaction process: EQU 2FeCl.sub.3 +FeS.sub.2 .fwdarw.3FeCl.sub.2 +S
While this process is interesting because it describes a method of removing pyritic sulfur, its principal disadvantage is that liberated solid sulfur must be separated from the coal solids. The solid sulfur is liberated by different methods such as froth flotation, vaporization and solvent extraction. All of these procedures, however, inherently add a second discrete process step with its attendent problems.
In another approach, U.S. Pat. No. 3,824,084 to Dillon, issued July 16, 1974, discloses a process involving grinding coal containing pyritic sulfur in the presence of water to form a slurry, and then heating the slurry under pressure in the presence of oxygen and sulfuric acid. The patent disloses that under these conditions the pyritic sulfur (for example, FeS.sub.2) can react to form sulfuric acid, ferrous sulfate and ferric sulfate. The patent discloses that typical reaction equations for the process at the conditions specified are as follows: EQU FeS.sub.2 +H.sub.2 O+7/2O.sub.2 .fwdarw.FeSO.sub.4 +H.sub.2 SO.sub.4 EQU 2FeSO.sub.4 +H.sub.2 SO.sub.4 +1/2O.sub.2 .fwdarw.Fe.sub.2 (SO.sub.4).sub.3 +H.sub.2 O
Numerous other methods have been proposed for reducing the pyritic sulfur content of coal and other carbonaceous material. For example, U.S. Pat. No. 4,155,717 to Sun, et al., discloses a process for reducing sulfur content of coal by treating coal particles with an aqueous solution of alkali metal and alkaline earth metal sulfites and bisulfites. Further, U.S. Pat. No. 4,233,034 to Miller, et al., discloses a process to reduce the amount of pyritic and organic sulfur in coal, shale and other carbonaceous materials by contacting these materials with hydrogen, transition metal salts and a buffer under elevated temperature and pressure.
While the art has provided a number of processes for the reduction of pyritic sulfur content of coal, shale and other carbonaceous materials, there still exists a need for methods to more effectively reduce the sulfur content of coal and other carbonaceous material.
Accordingly, it is one object of the present invention to provide a process for the elimination of pyrite in coal, shale and other carbonaceous materials.
It is another object of this invention to provide a method for the elimination or at least the substantial reduction of sulfur dioxide from the coal combustion products of coal, shale or other carbonaceous materials.
The achievement of these and other objects will be apparent from the following description of the subject invention.